Podzolic

Many would consider the Podzolic soils to be the most visually striking of Canadian soils. (Humo-Ferric Podzol 3) They are forested soils found primarily on sandy parent materials in areas underlain by igneous rocks, most prominently on the Canadian Shield, but are also found in other regions on sandy glacio-fluvial deposits. They are dominant on sandy deposits in ecozones or parts of ecozones where the mean annual precipitation is above about 700 mm. At mean annual precipitation levels below this Brunisolic soils are found on the same types of sandy deposits, most notably through NW Ontario and the Canadian Shield in the Prairie provinces. Coniferous-dominated plant communities are the major vegetation type found on Podzolic soils. (Humo-Ferric Podzol)

The sandy glacial sediments derived from igneous rocks typically have an acidic ph because of the mineralogical composition of the sediments. The acidity of the upper soil is further increased by the organic decomposition products from the coniferous leaf litter. This creates an intense chemical weathering zone in the upper part of the soil where primary minerals containing aluminum, iron, and other metal ions are weathered and the ions released into the soil solution. These metal ions form complexes with the organic decomposition products (also called chelates), and the complexes move with the vertically draining water into the B horizon where they are deposited. This is a brief summary of the complex and fascinating process of podzolization.

The intense weathering of the upper soil creates a very light gray Ae horizon, the e indicating that eluviation of various compounds has occurred. (Humo-Ferric Podzol 2) The Ae is underlain by a Podzolic B horizon , which is an illuvial horizon that has three main variants depending on the relative amount of organic compounds relative to iron. Although other ions like aluminum are also transferred within the profile during podzolization, the organic compounds and iron dominate the horizon colours (iron giving a reddish-brown colour and organic compounds a black colour). (Humo-Ferric Podzol)

Reddish-brown Podzolic B horizons have higher amounts of iron relative to organic matter and are classified as Bf horizons (iron = Fe). They would primarily be associated with the drier end of the Podzolic range (700 to 800 mm annual precipitation) and are commonly intermixed with Brunisolic soils.

At the wetter end of the Podzolic range such as the west coast of Vancouver Island (mean annual precipitation 1500 to 3000 mm) or in depressions throughout the Boreal forest, dark-coloured Podzolic B horizons dominated by organic matter occur. These are classified as Bh horizons. The Ae is often overlain by thick LFH layers or O layers. (Humic Podzol)

In between these two end points are Podzolic B horizons with high contents of organic matter and appreciable amounts of extractable iron and aluminum. These horizons are classified as Bhf horizons.

The specific criteria for the Podzolic B horizon are complex and involve both their appearance in the field (morphological criteria) and chemical tests that must be subsequently performed on the samples taken from the field. The main laboratory tests assess the amount of organic carbon present (the main measure of organic matter) and the amount of iron and aluminum that is bound to organic matter. The organically bound iron and aluminum is measured using a pyrophosphate extract, which breaks apart the metal-organic matter complex. The specific criteria for Bh, Bhf, and Bf horizons are based on different ratios between the organic C and pyrophosphate extractable iron and aluminum.

The deposition of the weathering products from the Ae horizon commonly leads to different degrees of cementation of layers within the soil profile. These cemented layers can form barriers to the vertical penetration of tree roots and of water, leading to water saturation of the layer above the cemented layer. Four types of cemented layers are found in Podzolic soils: Ortstein horizons, Duric horizons, Placic horizons, and fragipans. The first three are denoted with a c suffix (for example, Bhfc, Bfc, BCc) and fragipans are assigned an x suffix (Bx, BCx). (Fe Bands)

Podzolic Great Groups

The three great groups of the Podzolic order correspond to the three climatically related groups discussed above. The Humo-Ferric great group has a Bf horizon and occurs at the drier end of the Podzolic range. (Humo-Ferric Podzol) The Humic Podzol group is found at the wetter end of the range (either by region or landscape position) and has a Bh horizon. (Humic Podzol) The intermediate position in the range is occupied by the Ferro-Humic Podzol, which has a Bhf horizon. (Ferro-Humic Podzol) The specific criteria for each group is shown in the table below (from the Canadian System of Soil Classification, 3rd ed.)

Humic Podzol

Ferro-Humic Podzol

Humo-Ferric Podzol

B Horizon

Bh > 10cm thick

Bhf > 10cm thick

Bf, or thin Bhf + Bf > 10cm thick

Organic C

>1%

>5%

= 0.5-5%

Other

pyrophosphate Fe < 0.3%, pyrophosphate Fe >20

pyrophosphate Al + Fe >0.6% (> 0.4% for sands)

pyrophosphate Al + Fe (> 0.4% for sands)

Podzolic Subgroups

The subgroups of the Podzolic orders are based on a) presence of the cemented layers discussed above, b) transitions to other orders and c) soils lacking either a) or b).

Subgroup

Great Group

Humic

Ferro-Humic

Humo-Ferric

Orthic

X

X

X

Ortstein

X

X

X

Placic

X

X

X

Duric

X

X

X

Fragic

X

X

X

Luvisolic

Not applicable

X

X

Sombric

Not applicable

X

X

Gleyed

Not applicable

X

X

Gleyed Ortstein

Not applicable

X

X

Gleyed Sombric

Not applicable

X

X

Orthic subgroupSoils of the Orthic subgroup do not have any of the horizons discussed below in addition to the Podzolic B horizon.

Subgroups with cemented layers:

Ortstein subgroupSoils of this subgroup have an Orstein horizon (Bhc, Bfc) at least 3 cm thick.

Placic subgroupThese soils have a Placic (Bhfc, Bfc) horizon that can be either a single layer or multiple layers of thin (commonly < 5 mm) layers that are hard, impervious, and dark reddish brown to black and which may form a braided pattern as they join together and break apart into separate layers.

Duric subgroup:Unlike the previous two subgroups, soils of the Duric subgroup have a strongly cemented layer (BCc) that does not meet the criteria for a Podzolic B horizon. The colour of the Duric horizon is often very similar to that of the parent material.

Fragic subgroup:Soils of this subgroup have a fragipan (Bx or BCx) underlying the Podzolic B horizon.

Subgroups transitional to other orders:

Luvisolic subgroupThese soils have a Bt horizon that is greater in 50 cm depth from the mineral surface. If the Bt horizon is 50 cm or less from the surface they are classified into the Luvisolic order.

Sombric subgroupSoils of the Sombric subgroup have an Ah horizon at least 10 cm thick indicating mixing of the LFH layer into the mineral A horizon by earthworms and other burrowing organisms.

Gleyed subgroupThese soils have distinct to prominent mottles within 1 m of the mineral surface of the soil. The prefix can also be added to soils that meet the criteria of the Ortstein and the Sombric subgroups.P